Mathematics – Logic
Scientific paper
Feb 2003
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003georl..30c..28l&link_type=abstract
Geophysical Research Letters, Volume 30, Issue 3, pp. 28-1, CiteID 1128, DOI 10.1029/2002GL016074
Mathematics
Logic
3
Hydrology: Groundwater Transport, Hydrology: Instruments And Techniques, Geochemistry: Geochronology
Scientific paper
Quartz crystals in sandstones at depths of 1200 m-1400 m below the surface appear to reach a solubility equilibrium with the 4He-concentration in the surrounding pore- or groundwater after some time. A rather high 4He-concentration of 4.5 . 10-3 cc STP 4He/cm3 of water measured in a groundwater sample would for instance maintain a He pressure of 0.47 atm in a related volume. This value is equal within analytical error to the pressure deduced from the measured helium content of the quartz and its internal helium-accessible volume. To determine this volume, quartz crystals of 0.1 to 1 mm were separated from sandstones and exposed to a helium gas pressure of 32 atm at a temperature of 290°C for up to 2 months. By crushing, melting or isothermal heating the helium was then extracted from the helium saturated samples. A volume on the order of 0.1% of the crystal volume is only accessible to helium atoms but not to argon atoms or water molecules. By monitoring the diffusive loss of He from the crystals at 350°C an effective diffusion constant on the order of 10-9 cm2/s is estimated. Extrapolation to the temperature of 70°C in the sediments at a depth of 1400 m gives a typical time of about 100 000 years to reach equilibrium between helium in porewaters and the internal He-accessible volume of quartz crystals. In a geologic situation with stagnant pore- or groundwaters in sediments it therefore appears to be possible with this new method to deduce a 4He depth profile for porewaters in impermeable rocks based on their mineral record.
Gannibal Maria
Kalashnikov Evgenii
Kamensky Igor
Lehmann Bernhard E.
Pevzner Boris
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